Curious TOC problem

[Michael Barr, Prof.]

I am trying to compile a book.  There is no problem on the first pass.  On the second I get an error on line 2 of the .toc file (which is generated by the usual \tableofcontents macro).  The error message is totally unhelpful (perhaps a missing \item).  The first three lines of the .toc file are

\contentsline {chapter}{Preface}{v}{section*.1}
\contentsline {section}{\numberline {1}Definition of category}{1}{section.1}
\contentsline {subsection}{\numberline {1.1}Some constructions for categories}{2}{subsection.1.1}

and the error is on line 2.  If I change the first occurrence of  "section" to "subsection", that error goes away and then there is an error line 10 which is the toc entry for section 2.

What is going on?  What likes "chapter" and "subsection", but not "section"?  I am using TeXLive 2016 under Win-10.

Michael
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